FR: she washed it (her hair)

I was teaching the rule - elle s'est lavée but elle s'est lavé les cheveux ( object comes afterwards so reflexive pronoun is indirect object.
A student asked me how I would say she washed it and am unsure as have never seen this

elle se les est lavés ( agreement as PDO) however it looks completely wrong to use etre here with a PDO . I was tempted to try " elle les a lavés "
Can anybody help me here?

The rule remains the same: agreement with the preceding direct object if any.

Elle s'est lavée. (The direct object comes before the verb.)Elle s'est lavéles cheveux. (The direct object comes after the verb; the reflexive pronoun is therefore an indirect object and there is no agreement.)Elle se les est lavés. (The direct object comes before the verb; the reflexive pronoun is an indirect object.)Elle les a lavés. (The direct object comes before the verb.)